Chemical Reactions

CHEMICAL REACTIONSLecture -2

Non Reacting System

Sensible Heat Latent heat (phase change)Reacting System21 percent oxygen and 79 percent nitrogen by mole numbers.

EXAMPLE 2.1 Standard atmospheric conditions for many combustion calculations canbe represented by the following mole fractions:N2 78%O2 21%Ar 1%For these conditions, determine (a) the mass fractions of N2, O2, and Ar; (b) the mixture molecular weight, kg/kgmole; and (c) the specific gas constant R for air, J/kg K.

During a combustion process, the components that exist before the reaction are called reactants and the components that exist after the reaction are called products. Consider, for example, the combustion of 1 kmol of carbon with 1 kmol of pure oxygen, forming carbon dioxide, Combustion of CarbonThe fuel must be brought above its ignition temperature to start the combustion

The minimum ignition temperatures of various substances in atmospheric air are approximately 260C for gasoline, 400C for carbon, 580C for hydrogen, 610C for carbon monoxide, and 630C for methane. Chemical equations are balanced on the basis of the conservation of mass principle (or the mass balance), which can be stated as follows: The total mass of each element is conserved during a chemical reaction (Fig. 155). That is, the total mass of each element on the right-hand side of the reaction equation (the products) must be equal to the total mass of that element on the left-hand side (the reactants) even though the elements exist in different chemical compounds in the reactants and products.

EXAMPLE 151 Balancing the Combustion Equation

One kmol of octane (C8H18) is burned with air that contains 20 kmol of O2, Assuming the products contain only CO2, H2O, O2, and N2, determine the mole number of each gas in the products and the airfuel ratio for this combustion process.Solution The amount of fuel and the amount of oxygen in the air are given. The amount of the products and the AF are to be determined. Assumptions The combustion products contain CO2, H2O, O2, and N2 only.Properties The molar mass of air is Mair = 28.97 kg/kmol = 29.0 kg/kmolAnalysis The chemical equation for this combustion process can be written as

THEORETICAL AND ACTUAL COMBUSTION PROCESSES

The minimum amount of air needed for the complete combustion of a fuelis called the stoichiometric or theoretical air.

The theoretical combustion of methane isNotice that the products of the theoretical combustion contain no unburned methane and no C, H2, CO, OH, or free O2.

Dew-Point Temperature of Combustion ProductsEthane (C2H6) is burned with 20 percent excess air during a combustion process, as shown in Fig. 1511. Assuming complete combustion and a total pressure of 100 kPa, determine (a) the airfuel ratio and (b) the dew-point temperature of the products.

Solution The fuel is burned completely with excess air. The AF and the dew point of the products are to be determined.Assumptions 1 Combustion is complete. 2 Combustion gases are ideal gases.Analysis The combustion products contain CO2, H2O, N2, and some excess O2 only. Then the combustion equation can be written as

Reverse Combustion AnalysisOctane (C8H18) is burned with dry air. The volumetric analysis of the products on a dry basis is (Fig. 1513).

CO2: 10.02 percentO2: 5.62 percentCO: 0.88 percentN2: 83.48 percent

Determine (a) the airfuel ratio,(b) the percentage of theoretical air used, and (c) the amount of H2O that condenses as the products are cooled to 25C at 100 kPa.

Solution Combustion products whose composition is given are cooled to 25C. The AF, the percent theoretical air used, and the fraction of water vapor that condenses are to be determined.Assumptions Combustion gases are ideal gases.Properties The saturation pressure of water at 25C is 3.1698 kPa (Table A4).Analysis Note that we know the relative composition of the products, but we do not know how much fuel or air is used during the combustion process. However, they can be determined from mass balances. The H2O in the combustion gases will start condensing when the temperature drops to the dew point temperature.

The combustion equation for 1 kmol of fuel is obtained by dividing the above equation by 1.36,(a) The airfuel ratio is determined by taking the ratio of the mass of the air to the mass of the fuel

(b) To find the percentage of theoretical air used, we need to know the theoretical amount of air, which is determined from the theoretical combustionequation of the fuel,That is, 31 percent excess air was used during this combustion process. Notice that some carbon formed carbon monoxide even though there wasconsiderably more oxygen than needed for complete combustion.

(c) For each kmol of fuel burned, 7.37 + 0.65 + 4.13+ 61.38 + 9 =82.53 kmol of products are formed, including 9 kmol of H2O. Assuming thatthe dew-point temperature of the products is above 25C, some of the water vapor will condense as the products are cooled to 25C. If Nw kmol of H2O condenses, there will be (9 - Nw) kmol of water vapor left in the products.The mole number of the products in the gas phase will also decrease to 82.53 - Nw as a result. By treating the product gases (including the remaining water vapor) as ideal gases, Nw is determined by equating the mole fraction of the water vapor to its pressure fraction,Therefore, the majority of the water vapor in the products (73 percent of it)condenses as the product gases are cooled to 25C.

ENTHALPY OF FORMATION & ENTHALPY OF COMBUSTIONThe molecules of a system possess energy in various forms such as sensible and latent energy (associated with a change of state), chemical energy (associated with the molecular structure), and nuclear energy (associated with the atomic structure)

The microscopic form of energy of asubstance consists of sensible, latent,chemical, and nuclear energies.

During a chemical reaction, some chemical bonds that bind the atoms into molecules are broken, and new ones are formed. The chemical energy associated with these bonds, in general, is different for the reactants and the products. Therefore, a process that involves chemical reactions involves changes in chemical energies, which must be accounted for in an energy balance

When the existing chemical bonds are destroyed and new ones are formed during a combustion process, usually a large amount of sensible energy is absorbed or released.

however, the composition of the system at the end of a process is no longer the same as that at the beginning of the process. In this case it becomes necessary to have a common reference state for all substances. The chosen reference state is 25C (77F) and 1 atm, which is known as the standard reference state. Property values at the standard reference state are indicated by a superscript () (such as h and u).

The ideal-gas enthalpy of N2 at 500 K relative to the standard reference state, for example, is h

The combustion of carbon is an exothermic reaction (a reaction during which chemical energy is released in the form of heat). Therefore, some heat is transferred from the combustion chamber to the surroundings during this process, which is 393,520 kJ/kmol CO2 formed. (When one is dealing with chemical reactions, it is more convenient to work with quantities per unit mole than per unit time, even for steady-flow processes.)

The process described above involves no work interactions. Therefore, from the steady-flow energy balance relation, the heat transfer during this process must be equal to the difference between the enthalpy of the products and the enthalpy of the reactants.

This enthalpy change is different for different reactions, and it is very desirable to have a property to represent the changes in chemical energy during a reaction. This property is the enthalpy of reaction hR, which is defined as the difference between the enthalpy of the products at a specified state and the enthalpy of the reactants at the same state for a complete reaction.

For combustion processes, the enthalpy of reaction is usually referred to as the enthalpy of combustion hC, which represents the amount of heat released during a steady-flow combustion process when 1 kmol (or 1 kg) of fuel is burned completely at a specified temperature and pressure

The enthalpy of combustion is obviously a very useful property for analyzing the combustion processes of fuels. However, there are so many different fuels and fuel mixtures that it is not practical to list hC values for all possible cases.

The formation of CO2 during a steady flow combustion process at 25C and 1 atm.

The enthalpy of combustion represents the amount of energy released as a fuel is burned during a steady-flow process at a specified state.

The enthalpy of formation of a compound represents the amount of energy absorbed or released as the component is formed from its stable elements during a steady-flow process at a specified state

Another term commonly used in conjunction with the combustion of fuels is the heating value of the fuel, which is defined as the amount of heat released when a fuel is burned completely in a steady-flow process and the products are returned to the state of the reactants. In other words, the heating value of a fuel is equal to the absolute value of the enthalpy of combustion of the fuel.

Evaluation of the Enthalpy of Combustion Determine the enthalpy of combustion of liquid octane (C8H18) at 25C and 1 atm, using enthalpy-of-formation data from Table A26. Assume the water in the products is in the liquid form

Solution The enthalpy of combustion of a fuel is to be determined using enthalpy of formation data.

Properties The enthalpy of formation at 25C and 1 atm is -393,520 kJ/kmol for CO2, -285,830 kJ/kmol for H2O, and -249,950 kJ/kmol for C8H18 (Table A26).

Analysis The combustion of C8H18 is illustrated in Fig. 1520. The stoichiometric equation for this reaction isBoth the reactants and the products are at the standard reference state of 25C and 1 atm. Also, N2 and O2 are stable elements, and thus their enthalpy of formation is zero

which is practically identical to the listed value of 47,890 kJ/kg in Table A27. Since the water in the products is assumed to be in the liquid phase, this hC value corresponds to the HHV of liquid C8H18.

FIRST-LAW ANALYSIS OF REACTING SYSTEMS Steady-Flow Systems

A combustion chamber normally involves heat output but no heat input. Then the energy balance for a typical steady-flow combustion process becomes

Closed SystemsFor closed systems or a stationary chemically reacting closed system as